Recently, attention has been directed to the application of microalgae to the production of a variety of materials including lipids, hydrocarbons, oil, polysaccharides, pigments, and biofuels.
One of the conventional methods to grow microalgae is to heterotrophically culture it in an enclosed, light-free system. Techniques have been developed for the large-scale production of aquatic microalgae under heterotrophic growth conditions by utilizing organic carbon instead of light as an energy source. For example, U.S. Pat. Nos. 3,142,135 and 3,882,635 describe processes for the heterotrophic production of proteins and pigments from algae such as Chlorella, Spongiococcum, and Prototheca. In addition, heterotrophic algal cultures can attain much higher densities than photoautotrophic cultures.
However, the above application cannot be applied to all microalgae because only a limited number of microalgae strains can grow in heterotrophic conditions. Attempts to grow microalgae in heterotrophic conditions often involve either screening for strains that can grow in heterotrophic conditions or genetic modification of organisms to allow growth under such conditions.
Microalgae that contain both a proper transportation system for sugar and that can grow naturally in heterotrophic conditions often show slow growth rates or low production of materials of commercial interest since they have evolved many years to utilize sunlight as an environmental signal to control aspects of metabolism as well as energy generated through photosynthesis.
Most of the photosynthetic organisms, including microalgae, use light as an environmental signal to optimize themselves for growth and survival. Light signals are sensed by different photoreceptors including red/far-red photoreceptors (phytochromes) and blue light photoreceptors (cryptochromes and NPHs). Light serves as an environmental signal that regulates physiological and developmental processes and provides the energy that fuels the reduction of inorganic carbon. However, under certain conditions light also has the potential to be toxic. Photoinhibition occurs either when the photon flux absorbed by chloroplasts is very high (under high light conditions) or when the light energy influx exceeds the consumption capability (under mixotrophic conditions where a cell uses reduced carbon as an energy source). In mixotrophic conditions, photosynthetic organisms show photoinhibition at a much lower light intensity than autotrophic conditions since the electrons absorbed through the photosynthetic apparatus cannot be efficiently used due to a feedback mechanism in the Calvin cycle.
Absorbed light energy can result in the accumulation of excited chlorophyll molecules within the pigment bed and damage of the photosystem. Excited chlorophyll molecules that accumulate in the pigment bed as a consequence of excess excitation can also stimulate the production of active oxygen species such as superoxides, hydroxyl radicals and singlet oxygen.